Here are a couple videos that explain gravity and how things relate to each other. These should answer your question.
And here is an entire list of short courses on the universe that might interest you.
why does the planets stay on its orbit?
Because of two opposing forces upon planets, that of gravity around a nearby star or stars and the angular momentum (basically centrifugal force) of each planet. The two forces pull in opposite directions stablizing the orbits of local planets.